Hi guys, the Nixpkgs distribution [1] contains all of Hackage -- 7,898 packages at the time of this writing --, and we regularly build these packages with GHC 7.8.4 in our CI environment [2] running NixOS Linux. About 3,000 of these builds are disabled outright, because we know they won't succeed anyway (i.e. they have dependencies we cannot fulfill), but the remaining 5,000 packages compile successfully most of the time. Now, a few days ago, we created a separate build job [2] that compiles the same package set with GHC 7.10.1, and the result is interesting: with the newer compiler, approximately 1,500 packages are broken compared to the state with 7.8.4. A detailed list of broken packages can be found at [4]. Keep in mind that those figures only approximate reality. For instance, we patch packages sometimes to make their builds succeed even though the pristine version would not, and so far much more effort has gone into fixing 7.8.4 builds than we spent on fixing 7.10.1 builds. Also, some packages may compile fine when you run "cabal install <foo>" on your workstation, but those builds may fail in the context of NixOS for whatever reason. If anyone were to repeat this experiment on Darwin, Windows, Ubuntu, or Red Hat, the results would be different from our numbers! Still, based on the information we have, it's probably fair to say that the GHC 7.10.1 update breaks approximately 30% of all Haskell builds that worked with 7.8.4. Best regards, Peter [1] https://github.com/nixos/nixpkgs [2] http://hydra.cryp.to/jobset/nixpkgs/haskell-updates [3] http://hydra.cryp.to/jobset/nixpkgs/haskell-updates-ghc7101 [4] http://hydra.cryp.to/eval/304489#tabs-still-fail